With radio frequency synthesizers and especially with digital cellular telephones, it is desirable to have an ability to switch between analog transmission and digital transmission states. Present day analog radio systems, such as analog cellular telephone systems, are not able to transmit or receive digital signals because of the frequency switching requirements of digital transmission. In particular, the necessity in digital technology that a receiver or transmitter switch rapidly between different frequencies requires that the circuitry be able to rapidly stabilize or "lock" quickly onto a particular frequency.
The need for fast frequency switching capabilities is even more imperative in cellular telephone systems using multiplexed digital technology where the synthesizer is not only handling two basic communication channels (send and receive), but is also handling an overhead channel (where additional information is transmitted, such as location, signal strength, alternative cell sites, billing information, etc.) as well as at least one additional group of send, receive, and overhead frequencies. The additional group(s) of frequencies is monitored in order to determine when the communication link should be handed off to another cell site with better signal strength. Thus, an ability to switch frequencies is especially important in the field of digital cellular communications because the input reference signal frequency is frequently switched so that an alternative frequency signal from an alternative cell site can be checked.
Present day analog systems are simply unable to lock onto a new frequency quickly enough for digital transmission requirements. Frequency switching limitations in analog radios are imposed, in part, by the design of the phase lock loop circuits, which are used in radio synthesizers to compare an incoming reference signal to a local oscillating signal for the purpose of locking the local signal to the input reference signal and obtaining information from the phase shifts detected between the two signals. Thus, phase lock loop circuits are used in radio receivers to obtain information from an incoming reference signal having a particular frequency.
Prior art frequency synthesizers utilize the phase lock loop configuration shown in FIG. 1 which includes an input reference signal 1, a phase detector 2, a phase lock loop filter 3, and a voltage controlled oscillator (VCO) 4 coupled as shown in the drawing. Although there are a number of different configurations for phase lock loop filters, the description of the invention provided herein is given with reference to the basic phase lock loop filter configuration shown in FIG. 1 which includes a charge pump 3a and a second order R-C low pass filter 3b. Such a configuration is suitable for analog transmission, but is not suitable for frequency switching required in digital transmissions. Of course, the present invention can be implemented with loop filters having a variety of configurations which differ from the loop filter described herein.
It will be appreciated by those skilled in the art that the frequency switching capability of the phase lock loop circuit configuration shown in FIG. 1 is improved by improving (decreasing) the lock time, and that the lock time for the phase lock loop frequency synthesizer is improved by increasing the loop filter bandwidth W.sub.n of the phase lock loop circuitry. However, the loop filter bandwidth W.sub.n can be only increased to a point before negative resistors are required in the filter 3b.
Because the phase look loop filters as shown in FIG. 1 have only a limited ability to increase the bandwidth W.sub.n, present day analog synthesizers cannot meet the lock time, and therefore the frequency switching, requirements of digital transmission without the addition of complex circuitry or extensive reworking of the design.